Just nipping back to the Hornby Class 67 if I may briefly to add my thoughts ?
I have just fitted a Hornby TTS sound decoder to my Hornby Class 67 with one of your 20x40 bass reflex speakers.
I removed the screw mounting holes at either end of the speaker to enable fitment and then used the blu-tak method to secure the speaker and hold it still within the fuel tank enclosure, then soldered the speaker wires to the wires coming from the decoder, avoiding soldering anywhere near the actual decoder itself.
I was fortunate enough not to experience the slow speed stuttering reported elsewhere after fitting.
Now, after driving the real thing since their arrival in the UK, I hope I'm qualified enough to say that this speaker is ideal for the job in my honest opinion, it sounds just like a Class 67 should - I didn't feel it necessary to adjust any of the sound volumes or other settings at this time and the best bit was the ease of fitting the thing.
I also ordered two of your 'Mega-Bass' speakers in the same order and these will go in a ViTrains 47, but I'm not sure you really 'need' these in a Hornby 67, when the 20x40 does such a good job and without the effort of modifying anything ?
Thanks for your efforts on the forum and your youtube vids, sound fitted models have just become interesting again for me.
All the best,